1. Field of the Invention
The present invention relates to a mass spectrometric apparatus and more particularly to a technology to detect concealed explosives and illegal medicines using a mass spectrometric apparatus.
2. Description of the Background
As international disputes get more acute, a detector that can detect explosives is desired in order to prevent terrorism and maintain security. As such detectors, baggage inspection devices that make use of X-ray transmission are widely used, for example, at airports. A method that uses an X-ray detector or similar technology is known as “bulk detection” because an object is recognized as a bulk and a hazardous material is identified from the information such as the shape of the bulk.
In contrast, a detection method based on gas analysis is known as “trace detection,” and an object is identified by the chemical analysis information. A feature of trace detection is that it is possible to detect a very minute amount of a component adhering to a bag or other object. Under circumstances in which higher security is socially demanded, a device that can detect hazardous materials with a higher degree of accuracy by the combined use of bulk detection and trace detection is desired.
At the same time, detectors are presently utilized at customs and similar places in order to find illegal medicines (e.g., narcotics, drugs, or unregulated medicine) that are brought in through various routes. At customs, bulk detectors and drug-sniffing dogs are mainly used, and a trace analyzer for illegal medicine detection that can replace drug-sniffing dogs is desired.
In trace detection, various analytical methods have been posited such as ion mobility spectroscopy, gas chromatography, and similar processes. The development of a device simultaneously having speed (high throughput), sensitivity, and selectivity (low false alarm rate), all features that are important for a detector, is desired and described herein.
Under these conditions, mass spectrometry has very good throughput, sensitivity, and selectivity. Therefore, detection technologies based on mass spectrometry have been proposed (e.g., Japanese Patent Application No. JP-A No. 134970/1995, hereafter “Patent Document 1”).
FIG. 10 is a view explaining a detector of Patent Document 1 based on mass spectrometry. As shown in FIG. 10, an air introduction probe 1 is connected to an ion source 3 via an insulation pipe 2, and the ion source 3 is connected to an air suction pump 6 via a vent 4 and another insulation pipe 5. The ion source 3 is equipped with a needle electrode 7, a first aperture electrode 8, an intermediate pressure region 9, and a second aperture electrode 10. The needle electrode 7 is connected to a power supply 11, and the first and second aperture electrodes 8 and 10 are connected to an ion acceleration power supply 12. The intermediate pressure region 9 is connected to a vacuum pump via another vent 13. An electrostatic lens 14 is disposed at the back of the intermediate pressure region 9 and a mass analysis portion 15 and a detector 16 are disposed at the back of the electrostatic lens 14.
Detection signals from the detector 16 are fed to a data processing portion 18 via an amplifier 17. The data processing portion 18 identifies a plurality m/z (mass of ion/valence of ion) values showing a specific medicine and determines whether or not the specific medicine is contained in a test gas. The data processing portion 18 is equipped with a mass judgment portion 101, a substance A judgment portion 102, a substance B judgment portion 103, a substance C judgment portion 104, and an alarm operation portion 105. Further, display portions 106, 107, and 108 are disposed on an alarm display 19 operated by the alarm operation portion 105.
In hazardous material detection based on mass spectrometry, ion sources particularly suitable for substances, such as explosives, that tend to be negatively ionized have been reported (e.g., Japanese Patent Application No. JP-A No. 93461/2001, hereafter “Patent Document 2”). FIG. 11 is a drawing showing an example of an ion source according to Patent Document 2 in a detector using mass spectrometry.
In FIG. 11, a sample introduced through a sample introduction tube 20 is then introduced into an ion drift region 21. Part of the sample introduced into the ion drift region 21 is introduced into a corona discharge portion 22 and the rest of the sample is ejected to the exterior of the ion source through a sample exhaust tube 23. The part of the sample introduced into the corona discharge portion 22 is fed into a corona discharge region 25 that is formed at the tip of a needle electrode 24 by imposing a high voltage and ionized. At that time, the sample is introduced from the direction nearly opposing the direction of the flow of ions drifting from the needle electrode 24 toward a counter electrode 26. The generated ions are introduced into the ion drift region 21 through the opening 27 of the counter electrode 26 by an electric field.
At this time, it is possible to make the ions drift and efficiently introduce them into a first aperture 28 by imposing a voltage between the counter electrode 26 and the first aperture 28. The ions introduced through the first aperture 28 pass through a second aperture 29 and a third aperture 30 where they are analyzed with a mass spectrometer (not shown in the figure) disposed at a vacuum region 31.
The flow rate of the sample introduced into a corona discharge region 25 is important for a stable ionization and therefore a suction tube 32 and a flow controller 33 are connected to the corona discharge portion 22. The rate of flow passing through the sample exhaust tube 23 and the suction tube 32 is determined by the flow controller 33, the capacity of a suction pump 34, and the conductance of various tubes including the sample introduction tube 20.
Similarly, the following technologies related to mass spectrometry are known in the art:
(1) Providing two needle electrodes for an ion source and discharging ions with the polarities of two needle electrodes being different from each other, one being positive and the other being negative (Japanese Patent Application No. JP-C No. 516140/2001, hereafter “Patent Document 3”);
(2) Providing two mass spectrometers side-by-side, physically separating (i.e., ramifying) a sample gas into two portions, introducing each of the separated sample gas portions into the two mass spectrometers, and measuring positive ions with one of the mass spectrometers and negative ions with the other mass spectrometer (Japanese Patent Application No. JP-A No. 236582/1997, hereafter “Patent Document 4”);
(3) Disposing two needle electrodes to an ion source, one at a position suitable for positive ionization and the other at a position suitable for negative ionization, and switching the ionizing polarity, wherein a positive high voltage is applied on both of the needle electrodes in the event of positive ion measurement and a negative high voltage is applied on both of the needle electrodes in the event of negative ion measurement (Japanese Patent Application No. JP-A No. 181783/2002, hereafter “Patent Document 5”);
(4) An ion source having a plurality of needle electrodes (Japanese Patent Application No. JP-A No. 351569/2001, hereafter “Patent Document 6”); and
(5) Providing two ion sources for corona discharging and electrostatic spraying, generating ions having polarities different from each other, introducing the ions having each polarity into an identical vacuum region through each aperture, and blending the positive and negative ions in the vacuum region (Japanese Patent Application No. JP-A No. 242926/2003, hereafter “Patent Document 7”).
In the method of using corona discharge, known as the “atmospheric pressure chemical ionization method,” there are two types: (1) a mode of generating positive ions; and (2) a mode of generating negative ions. The appropriate mode depends upon the object to be measured. For example, since most illegal medicines such as narcotics and stimulant drugs have a high proton affinity in a vapor phase, they are highly efficient in generating proton-added positive pseudo molecular ions in the positive ionization mode. In contrast, in the case of nitro-compounds into which most explosives are classified, since they have a high electro-negativity, they are ionized with a high degree of efficiency in the negative ionization mode.
In the case of use in an airport, up to the present time, the object of security staff has been to detect explosives and the object of customs staff has been to find illegal medicines such as narcotics and stimulant drugs. That is, it has been well accepted to use the negative ionization mode for detection by security staff and to use the positive ionization mode for detection by customs staff, Either mode has been widely used depending on the users, locations of use, and status of use. However, in view of the recent social situation, the desire to simultaneously detect explosives and illegal medicines has intensified. For example, even at customs, the need to prevent smuggling of not only illegal medicines but also explosives that may possibly be used in domestic terror is increasing.
In the case of the detector according to Patent Document 1, since the positive ionization mode and the negative ionization mode are used alternately, it has been difficult to detect a substance suitable for the positive ionization mode and a substance suitable for the negative ionization mode simultaneously, with a high sensitivity.
In the case of an analyzer, for example a device made by directly connecting liquid chromatograph to a mass spectrometer, the time spent while a sample is transferred from the liquid chromatograph to the mass spectrometer is in the range of about one minute. Therefore, it is well within the capacity of measuring positive ions and negative ions substantially at the same time while the polarity of the ion source and the mass spectrometer is switched every few seconds. Nevertheless, when it is attempted to apply this method to the field of hazardous material detection, there arise several problems and the application is substantially difficult. The problems arise because, under some detection conditions, an object gas to be measured reaches an ion source only for a very short period of time, such as from 0.2 to 0.3 sec. To cope with that situation, it becomes necessary to measure an object gas while the polarity of discharge is reversed at a high speed, for example the polarity may be switched every 0.05 sec.
However, a challenge of the method has been that, when measurement is carried out while the polarity of high voltage applied on a needle electrode is switched at such a high speed, the corona discharge generated at the tip of the needle electrode is not sufficiently stabilized and the reproducibility of the detection result deteriorates. Further, there is a possibility that an electric current load of about 1 mA is instantaneously applied on a high voltage power supply in accordance with the high speed reversal of polarity. Moreover, when the polarity is reversed every 0.05 sec., the electric current load is incurred more than two million times even with 24 hour continuous operation of the device. Therefore, another problem has been that a high voltage power source that can withstand such shocks has not been developed, and especially not at a reasonable cost.
Furthermore, since dischargeability varies in accordance with the polarity imposed on a needle electrode, a problem arises that, when the tip of the needle electrode is damaged by discharge, discharge of either of the positive or negative polarity does not normally occur in some cases. Therefore frequent maintenance, such as to change of the needle electrode, is necessary and the timing of such is difficult to detect.
For the aforementioned reasons, a detector capable of simultaneously detecting illegal medicines that are compatible with positive ionization and explosives that are compatible with negative ionization has long been desired.